This invention relates to a feedforward type circuit structure with programmable zeroes for synthesizing time-continual filters, delay chains, and the like, which structure comprises a pair of amplification cells interconnected at at least one interconnection node and connected between a first signal input of a first of said cells and an output terminal of the second of said cells, each cell in the pair comprising a pair of transistors which have a conduction terminal in common and have the other conduction terminals coupled respectively to a first voltage reference through respective bias members.
Specifically, the invention relates to a circuit structure as above adapted to implement programmable delay cells having a broad pass-band and flat group delay. The description which follows will cover this specific field of application for convenience of illustration only.
As is well known, the frequency response of an amplified electric signal bears a relation of amplitude, or modulo, to phase of the signal.
More particularly, as the band expands the phase changes, and the so-called group delay diminishes accordingly. Group delay is the derivative of phase with respect to frequency.
In theory, a programmable delay of infinite bandwidth would be ideal.
In practice, this cannot be achieved using prior art circuits. A programmable delay to meet an applicational requirement, and a sufficiently broad band, can be obtained at most.
Conventional circuits designed to fill the demand have used operational amplifiers, which require feedback control loops and filters of a high order, exhibit high power consumption, and occupy large circuit areas.
In conventional feedback circuit structures, the differentiated signals are input to high-frequency circuit nodes, introducing xe2x80x9czeroesxe2x80x9d in the transfer function and giving rise to the above-noted drawbacks.
In an effort to obviate such drawbacks, a circuit structure has been proposed in the prior art which is of the feedforward type with programmable xe2x80x9czeroesxe2x80x9d. This prior structure can synthesize time-continual filters by expanding the signal band without appreciably distorting the group delay.
Such a structure is described in European Patent Application No. 97830696.7 by the Applicant, for example.
While in many ways advantageous and essentially achieving its objectives, the structure disclosed in the above patent application has first and second feedforward circuit legs which introduce a total of three xe2x80x9cpolesxe2x80x9d, and only one xe2x80x9czeroxe2x80x9d, in the transfer function.
The greater the offset, the greater the delay obtained. That structure represents something of a frequency booster expanding the band.
This is achieved at the expense of some circuit complexity and enhanced power consumption.
According to principles of the present invention, a circuit structure of the feedforward type with programmable zeroes, particularly for synthesizing time-continual filters, delay chains and the like is provided. The structure has uniquely simple constructional and functional features effective to produce augmented group delay for a very moderate use of power, thereby overcoming the limitations of prior solutions.
One object of the invention is to provide a circuit structure of the feedforward type with programmable zeroes, for synthesizing time-continual filters, delay chains and the like, which is highly reliable in operation and relatively inexpensive to manufacture.
A feedforward type of circuit leg is connected to the first cell of the structure to the output terminal, the feedforward circuit leg being effective to introduce a xe2x80x9czeroxe2x80x9d in the right-hand semiplane of the pole-zero complex plane.
Based on this principle, the circuit has a circuit leg to connect a node of the first cell to said output terminal, said circuit leg comprising a transistor which has a control terminal connected to said node of the first cell, a first conduction terminal connected to said output terminal, and a second conduction terminal coupled to a second voltage reference through a capacitor.
The features and advantages of a circuit structure according to the invention will be apparent from the following description of an embodiment thereof, given by way of example and not of limitation with reference to the accompanying drawings.